1. Field of the Invention
The present invention relates to a positive-displacement type pump system, such as a fixed-cylinder type radial piston pump system, whose delivery volume can be adjusted depending upon requirement for an actuator to be operated with a pressurized working fluid discharged from the pump system.
2. Description of the Related Art
Such a pump system is disclosed, for example, in Japanese Patent Application Laid-open Publication No. 1-262,374, and typically includes a common pump housing formed with a plurality of cylinders, a plurality of pistons slidably received in the respective cylinders and cooperating therewith to form a plurality of pump units, an eccentric drive shaft extending into the housing for driving the pistons into reciprocating motion within the respective cylinders, and a common suction passage between a working fluid source and the pump units.
In order to minimize the energy consumption for driving the pump system, the pump system should discharge the minimum quantity or volume of working fluid required for one or more actuators to be operated by the working fluid which has been pressurized by the system. For enabling an adjustment of the delivery volume of the pump system as required from time to time depending upon requirement for the actuator, a typical approach would be to adjust the eccentricity of the drive shaft. However, such an approach necessarily makes the entire system complex in structure and less reliable in operation. Thus, in view of these drawbacks, the publication cited above further discloses the provision of a throttle valve disposed in the suction passage between the working fluid source and the pump units, for adjusting the flow rate of the working fluid from the working fluid source to be supplied to the pump units.
When the pump units of a single pump system are classified into a plurality of pump unit groups for individually supplying pressurized working fluid to different actuators, there may be a situation where it is only for one group of the pump units whose delivery volume is to be adjusted. On such occasion, however, the adjustment by means of the throttle valve disposed in the common suction passage is inevitably influential on the delivery volume of another group of the pump units for which delivery volume should not be adjusted. It has therefore been considered impossible to apply the unique teachings of the aforementioned publication for individually adjusting the delivery volume of the pump units of the different groups.